1. Field of the Invention
This invention generally relates to light harvesting porphyrin polymer materials and, more particularly, to a method for functionalizing a substrate with a metalloporphyrin polymer.
2. Description of the Related Art
Natural photosynthesis is the process that converts visible light into chemical energy. Chlorophyll a and b function as the dominant molecular light-harvesting “absorbers” in the photosynthetic reaction center (PRC) and exhibit complementary absorption characteristics in the wavelength range of less than 400 to about 700 nanometers (nm). Overall, the process is initiated with the absorption of light by a photosynthetic chromophore that functions as a molecular antenna for harvesting light. Cyclic tetrapyrroles (i.e., chlorophylls and porphyrins) act as efficient antennas due to their ability to absorb light throughout the visible spectrum. Noteworthy is the fact that the unique, three-dimensional architecture of the PRC permits the initial photophysical events to proceed with quantum efficiencies approaching unity.
FIG. 1 is a simplified schematic of a dye-sensitized solar cell (DSC), depicting modes of operation (prior art). In many respects, DSCs operate analogously to photosynthesis and are commonly referred to in terms of artificial photosynthesis. The modes of operation are: (1) following photoexcitation (light absorption), the adsorbed photosensitizer in its ground state (S) is promoted to an electronically excited state (S*) from which electron injection into the conduction band of a large band semiconductor (TiO2) occurs; (2) injected electrons diffuse through the TiO2 before reaching the anode and external circuit of the cell; (3) the charge-neutral state of the photosensitzer is regenerated through a transfer of positive charge from the photosensitizer cation (S+) to a liquid electrolyte following reaction with a redox couple in the solution (typically I−/I3−); (4) finally, I3− is reduced back to I− at the counter electrode (cathode).
FIGS. 9A and 9B respectively depict schematics of porphyrin and metalloporphyrin molecules (prior art). Porphyrins are heterocyclic macrocycles composed of four modified pyrrole subunits (a five-membered ring with the formula C4H4NH) interconnected at their a carbon atoms via methine bridges (═CH—), see FIG. 9A. Porphyrins are the conjugate acids of ligands that bind metals (M) to form complexes. The metal ion usually has a charge of 2+ or 3+, see FIG. 9B.
Although chlorophyll and its derivatives have diverse chemical structures, they exhibit characteristic absorption properties over broad wavelength ranges. In addition to their structural resemblance to natural chromophores such as chlorophyll a, synthetic porphyrins are attractive candidates as light-harvesters for photovoltaic (PV) applications due to their high structural stability, light absorption capabilities in the visible region, advantageous redox properties and synthetic accessibility as compared to naturally occurring chromophores. Investigations of photo-induced processes involving porphyrins have focused primarily on free-base (H2P) and zinc porphyrins (ZnP), both of which exhibit appreciably high quantum yields. In addition, the lifetimes of their respective photo-excited states are sufficiently long to allow electron transfer (ET) to compete with internal decay processes. Electron transfer involving porphyrins is facilitated by the highly delocalized π-system, which is capable of resisting major structural changes upon oxidation. Overall, the redox properties of porphyrins and metalloporphyrins are dramatically altered upon photo-excitation, which leads to the generation of porphyrin excited states that are advantageous in PV applications.
At the present, ruthenium(II) complexes have historically proven to be among the most efficient sensitizers in dye-sensitized solar cells.2 However, only incremental improvements in the highest power efficiencies have been achieved within the past decade. Considering the facts that ruthenium(II) dyes are expensive and ruthenium itself is a rare metal, there exists significant motivation for developing novel photosensitizers that either contain abundant, inexpensive metals or no metals at all.
The ability of porphyrins to efficiently harvest light over broad wavelength ranges has generated significant interest in their potential as light absorbing materials in solar applications over the last few decades. With respect to DSC, Yella et al. reported an exceptional power conversion efficiency (PCE) of 12.3% for a zinc porphyrin (YD2-o-C8) using co-sensitization with an organic dye (Y123) with Co(II/III) as redox couple.3 Overall, synthetic protocols towards the fabrication of “customized” porphyrin architectures have become well-established and have been widely adopted as conventional methods. In general, the absorption/electronic properties of porphyrins can be readily altered using a number of strategies including the following: functionalization along the porphyrin periphery, extension of π-conjugation, insertion of transition metals into the macrocyclic core, and complexation of metalloporphyrins with various ligands, among others.
Often, the desired optical and/or electronic enhancements accessible through chemical modification of porphyrins involve manipulation of light-harvesting (absorption) capabilities and/or excited-state behaviors (electron transfer, for example). Indeed, it is well-known that increasing the π-conjugation extending from the porphyrin core can lead to enhanced absorption properties which may include: (1) increased absorptivity over a particular wavelength range, (2) a broadening of optical absorption over wider ranges and/or (3) shifting of absorption towards longer (or shorter) wavelengths. Not surprisingly, this can be accomplished through a number of synthetic strategies and may involve simple attachment of conjugated moieties to the porphyrin core unit, formation of larger, porphyrin-centered macrocycle derivatives accessible through ring-fusion reactions, or linear (or branched) poly-porphyrin architectures through which networks (dimers, trimers, tetramers, polymers) of porphyrin subunits are connected through various motifs. Most often, porphyrin dimers, trimers, and tetramers are constructed in a step-wise approach whereby single porphyrin units are added to the evolving tetramer. In these cases, the construction of the final porphyrin arrays involves: (1) synthesis of monomeric porphyrin subunits (or multiple subunits), (2) chemical reaction(s) to “link” two individual subunits, (3) purification of the new molecule and, finally, (4) a repeat of steps (1)-(3). Although precise control over subunit reactivity and overall configuration is possible, this approach is both challenging and time-consuming, while often leading to only modest quantities of final porphyrin material. Nevertheless, elaborate porphyrin architectures can indeed been realized using these methodologies.
As opposed to the integration of single (monomeric) porphyrin-based photosensitizers employed in the case of DSC applications, the fabrication of extended networks(dimers/tetramers/polymers) of porphyrin-based materials is a promising strategy for realizing strongly, broadly absorbing architectures for light absorbing applications. Indeed, a number of publications describe the enhanced absorption characteristics of extended porphyrin arrays, the most relevant of which are briefly summarized below within the context of practical application where appropriate.
Lin et al. described the synthesis and photophysical studies of conjugated acetylenic porphyrin arrays prepared through either metal-mediated and/or organometallic coupling reactions of porphyrin monomers.4 As a result, the optical absorption properties of the dimeric (and trimeric) porphyrin arrays were significantly enhanced with respect to the corresponding monomeric porphyrins. Wagner et al. described the synthesis of phenylacetylene and phenylbutadiyne-linked porphyrin dimers and trimers using mild, copper-free conditions.5 Therein et al. described the preparation of highly-conjugated porphyrin arrays using metal-mediated cross-coupling reactions with metalloporphyrins.6 
In addition to the methodologies described above, Chen et al. reported the synthesis of porphyrin-containing polymers linked through triazole rings.7 Jyothish et al. provided the preparation of a series of tris(arylmethyl)ammonium-coordinated molybdenum(VI) propylidyne catalysts that enabled the efficient synthesis of ethynylene-bridged porphyrin-based arylene ethynylene polymers through alkyne metathesis.8 Xiang et al. described the synthesis of two conjugated polymers consisting of alternating main chain structures of zinc porphyrin-terthiophene (P-PTT) and zinc porphyrin-oligothiophene (P-POT).9 Shi et al. provided a donor-acceptor porphyrin-containing conjugated co-polymer (PCTTQP) that exhibited broad absorption along the visible spectrum.10 Natori et al. reported the synthesis of a tetraphenylporphyrin-end-functionalized poly(p-phenylene) exhibiting a well-controlled polymer chain structure and broad absorption across the visible region.11 Finally, Wirotius et al. described the fabrication of dendrimer-like star-branched poly(ethylene oxide)s (PEOs) comprising two and three generations with zinc tetraphenylporphyrin (ZnTPP) moieties located at both the core and at each branching point through a convergent approach using “click” chemistry.12 
FIGS. 2A and 2B respectively depict the molecular structures of porphyrin and porphyrin-[60]fullerene polymers fabricated via electropolymerization (prior art). An alternative to the various approaches for the fabrication of porphyrin polymers described above entails direct electrochemical polymerization of the appropriate porphyrin subunits (monomers). Giraudeau et al. demonstrated a convenient method for the electropolymerization of porphyrins that circumvents the difficulties associated with synthesizing a functional porphyrin monomer by directly employing a commercial zinc-β-octaethylporphyrin (ZnOEP) with 4,4′-bipyridine (bpy) in solution.13 Gust et al. reported the in situ electropolymerization of porphyrins and porphyrin-[60]fullerene dyads to form conjugated porphyrin (and porphyrin-[60]fullerene-based) materials.14-17 Overall, the absorption characteristics of the films resemble that of the corresponding porphyrin subunits, respectively, except for significant broadening of absorption peaks due to extended conjugation in the polymers.
FIGS. 3A through 3C respectively depict porphyrin dimers consisting of two free-base porphyrins (FIG. 3A), two zinc porphyrins (FIG. 3B), and one free-base porphyrin and one zinc porphyrin (FIG. 3C), whereby Im=imidazole (prior art). In contrast to methodologies that furnish extended, covalently-bonded porphyrin networks, the formation of porphyrin polymers has been successfully demonstrated through “self-assembly” of porphyrin monomers in solution. Michelsen et al. demonstrated the ability to control the self-assembly of polymers formed from cobalt porphyrins functionalized with two pyridine ligands.18 Kobuke et al. described the design and synthesis of alkyne-linked, bis(imidazoyl)porphyrin complex (dimer) arrays that exhibited large, dual photon absorption characteristics.19 
For some applications, it may be advantageous to provide an intimate association/communication between a porphyrin (polymer) and a substrate. For example, the substrate material may be a metal oxide such as TiO2 (as in DSC), and a pristine metal nanoparticle or various forms/compositions of nanostructures that have been either deposited on a substrate or dispersed within a matrix. Conventionally, this association/communication may be accomplished by chemically installing a functional linker on the porphyrin polymer, substrate, or sometimes both. In effect, the linker promotes a degree of interaction (electronic or other) between the porphyrin polymer and substrate which is not similarly operative in the isolated systems. Ozawa et al. demonstrated the synthesis of conjugated gold nanoparticle-terminated porphyrin polymers through the reaction of thiol-terminated porphyrin polymers with gold nanoparticles.20 Parsons provided a solar cell including a substrate having a horizontal surface and an electrode layer comprising a plurality of vertical surfaces, to which light-harvesting rods were coupled.21 Furthermore, the light harvesting rods consist of extended porphyrin networks fabricated by thermally-facilitated polymerization of porphyrin subunits, where polymerization originated from the terminus of porphyrins that had been pre-deposited in the form of a self-assembled monolayer (SAM). Optionally, the porphyrin subunits may contain additional chemical functionalities that provide attachment to quantum dots. Finally, Bocian et al. provided a method for fabricating redox-active polymers attached to surfaces.22 In certain embodiments, porphyrins (and/or metallocenes) are attached to a conductive substrate and can be oxidized upon applied voltage and subsequently store charge when the potential is removed, thereby providing the foundation of a memory storage device.
It would be advantageous to provide a substrate functionalized with a metalloporphyrin polymer in a more straightforward process than has been thus far demonstrated in the prior art.    1. F. O. Lenzmann and J. M. Kroon, “Recent Advances in Dye-Sensitized Solar Cells”, Advances in OptoElectronics 2007, Article ID 65073.    2. C-Y. Chen, M. Wang, J-Y. Li, N. Pootrakulchote, L. Alibabaei, C-h. Ngoc-le, J-D. Decoppet, J-H. Tsai, C. Grätzel, C-G. Wu, S. M. Zakeeruddin and M. Grätzel, “Highly Efficient Light-Harvesting Ruthenium Sensitizer for Thin-Film Dye-Sensitized Solar Cells”, ACS Nano 2009, 3, 3103-3109.    3. A. Yella, H-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W-G. Diau, C-Y. Yeh, S. M. Zakeeruddin and M. Grätzel, “Porphyrin-Sensitized Solar Cells with Cobalt (II/III)-Based Redox Electrolyte Exceed 12 Percent Efficiency”, Science 2011, 334, 629-633.    4. V. S-Y. Lin, S. G. DiMagno and M. J. Therien, “Highly Conjugated, Acetylenyl Bridged Porphyrins: New Models for Light-Harvesting Antenna Systems”, Science 1994, 264, 1105-1111.    5. R. W. Wagner, T. E. Johnson, F. Li and J. S. Lindsey, “Synthesis of Ethyne-Linked or Butadiyne-Linked Porphyrin Arrays Using Mild, Copper-Free, Pd-Mediated Coupling Reactions”, Journal of Organic Chemistry 1995, 60, 5266-5273.    6. M. J. Therien and S. G. DiMagno, “Metal-Mediated Cross-Coupling with Ring-Metalated Porphyrins”, U.S. Pat. No. 5,986,090, 1999.    7. H. Chen, J. Zeng, F. Deng, X. Luo, Z. Lei and H. Li, “Synthesis and Photophysical Properties of Porphyrin-Containing Polymers”, Journal of Polymer Research 2012, 19:9880.    8. K. Jyothish, Q. Wang and W. Zhang, “Highly Active Multidentate Alkyne Metathesis Catalysts: Ligand-Activity Relationship and Their Applications in Efficient Synthesis of Porphyrin-Based Aryleneethynylene Polymers”, Advanced Synthesis & Catalysis 2012, 354, 2073-2078.    9. N. Xiang, Y. Liu, W. Zhou, H. Huang, X. Guo, Z. Tan, B. Zhao, P. Shen and S. Tan, “Synthesis and Characterization of Porphyrin-Terthiophene and Oligothiophene π-Conjugated Copolymers for Polymer Solar Cells”, European Polymer Journal 2010, 46, 1084-1092.    10. S. Shi, X. Wang, Y. Sun, S. Chen, X. Li, Y. Li and H. Wang, “Porphyrin-Containing D-π-A Conjugated Polymer with Absorption Over the Entire Spectrum of Visible Light and Its Applications in Solar Cells”, Journal of Materials Chemistry 2012, 22, 11006-11008.    11. I. Natori, S. Natori, A. Kanasashi, K. Tsuchiya and K. Ogino, “Synthesis of Porphyrin-End-Functionalized Poly(p-phenylene) as a Leaf-Green-Colored Soluble Semiconducting Polymer: Control of π-π Interactions for Visible-Light Absorption”, Reactive and Functional Polymers 2013, 73, 200-206.    12. A-L. Wirotius, E. Ibarboure, L. Scarpantonio, M. Schappacher, N. D. McClenaghan and A. Deffieux, “Hydrosoluble Dendritic Poly(ethylene oxide)s with Zinc Tetraphenylporphyrin Branching Points as Photosensitizers”, Polymer Chemistry 2013, 4, 1903-1912.    13. A. Giraudeau, D. Schaming, J. Hao, R. Farha, M. Goldmann and L. Ruhlmann, “A Simple Way for the Electropolymerization of Porphyrins”, Journal of Electroanalytical Chemistry 2010, 638, 70-75.    14. P. A. Liddell, M. Gervaldo, J. W. Bridgewater, A. E. Keirstead, S. Lin, T. A. Moore, A. L. Moore and D. Gust, “Porphyrin-Based Hole Conducting Polymer”, Chemistry of Materials 2008, 20, 135-142.    15. M. Gervaldo, P. A. Lidell, G. Kodis, B. J. Brennan, C. R. Johnson, J. W. Bridgewater, A. L. Moore, T. A. Moore and D. Gust, “A Photo- and Electrochemically-Active, Porphyrin Fullerene Dyad Electropolymer”, Photochemical & Photobiological Sciences 2010, 9, 890-900.    16. J. D. Gust, P. A. Liddell, M. A. Gervaldo, J. W. Bridgewater, B. J. Brennan, T. A. Moore and A. L. Moore, “Electrically Conducting Porphyrin and Porphyrin-Fullerene Electropolymers”, US2010/0065123 A1.    17. B. J. Brennan, P. A. Lidell, T. A. Moore, A. L. Moore and D. Gust, “Hole Mobility in Porphyrin and Porphyrin-Fullerene Electropolymers”, Journal of Physical Chemistry B 2013, 117, 426-432.    18. U. Michelsen and C. A. Hunter, “Self-Assembled Porphyrin Polymers”, Angewandte Chemie International Edition 2000, 39, 764-767.    19. Y. Kobuke and K. Ogawa, “Porphyrin Array Exhibiting Large Two Photon Absorption Property and Including, as Structural Unit, Bis(imidazolylporphyrin Metal Complex) Linked with Acetylenic Bond and the Derivative Thereof, and Method of Producing the Same”, U.S. Pat. No. 7,022,840 B2, 2006.    20. H. Ozawa, M. Kawao, H. Tanaka and T. Ogawa, “Preparation of Long Conjugated Porphyrin Polymers with Gold Nanoparticles at Both Ends as Electronic and/or Photonic Molecular Wires”, Chemistry Letters 2009, 38, 542-543.    21. G. Parsons, “Nano-Structured Photovoltaic Solar Cell and Related Methods”, U.S. Pat. No. 7,655,860 B2, 2010.    22. D. F. Bocian, Z. Liu and J. S. Lindsey, “Procedure for Preparing Redox-Active Polymers on Surfaces”, U.S. Pat. No. 8,231,941 B2, 2012.